1. Field of the Invention
The present invention relates to a lighting unit, and more particularly, to a backlight unit and a method of driving the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for preventing parasitic capacitance from a lamp positioned at a lower end of the backlight unit and to allow quick initial driving of a lamp.
2. Discussion of the Related Art
A cathode ray tube (CRT) is a related art display device that has been used as a monitor in TV sets, metering devices, information terminals and the like. However, the CRT has a large size and a heavy weight. Because the current trend in electronics has been to reduce size and weight, various types of display devices have been investigated to replace the CRT. More specifically, flat panel display devices, such as liquid crystal display (LCD) devices using an electro-optic effect, plasma display panel (PDP) devices using gas discharge, electroluminescence display (ELD) devices using an electric field light emitting effect, have been investigated to replace the CRT. Among these flat panel display devices, the LCD device has the advantages of light weight, thin profile and low power consumption. Thus, LCD devices are used as monitors for desk-top computers and large size information display devices as well as monitors for lap-top computers. Since most LCD devices are light receiving type devices, which display an image by regulating quantity of light entering from a light source is needed. A backlight is typically used as a light source for providing light to an LCD panel.
Generally, the backlight unit, which is used as the light source of an LCD device, includes at least one cylindrical fluorescent lamp as a light source. Backlight units can be classified as either an edge light type or a direct light type. In the edge light type backlight unit, a lamp unit is positioned at one side of a light guide plate, which guides the light across the LCD panel, a lamp to emit light within the lamp unit, lamp holders at both sides of the lamp within the lamp unit, and a lamp reflection plate within the lamp unit to reflect light emitted from the lamp towards the light guide plate. The edge type backlight unit with the lamp unit positioned at the side of the light guide plate is used in relatively small-sized liquid crystal display device, such as monitors for lap-top computers and desk-top computers. The edge type backlight unit provides good light uniformity, long endurance, and has a thin profile.
The direct light type has been developed so as to be able to provide enough light to an LCD device having a size of 20 inches or more. More specifically, the direct light type has a plurality of light sources positioned on a lower surface of a diffusion plate such that light is directly illuminated toward a rear side of an LCD panel. Since the direct light type has a higher light utilization efficiency than the edge light type, the direct light type is used for large LCD devices, which require a large amount of light. However, the direct light type backlight unit for use in an LCD device of a large monitor or a large TV screen has many light sources. Further, the light sources are used for a long period of time. Thus, the image quality of such LCD devices suffer when a light source goes out due to defects in the or burns out from long time use.
An electroluminescent lamp (EL), a light emitting diode (LED), a cold cathode fluorescent lamp (CCFL), a hot cathode fluorescent lamp (HCFL), or an external electrode fluorescent lamp (EEFL) may be used as a light source in either the edge type backlight unit or the direct light type backlight unit.
FIG. 1 is a schematic perspective view illustrating a related art light type backlight unit. As shown in to FIG. 1, a related art backlight unit includes a plurality of lamps 1, a case frame 3 to secure and hold the lamps 1, and light scattering members 5a, 5b and 5c disposed between the lamps 1 and a liquid crystal panel (not shown). The case frame 3 has a reflection sheet 7 disposed on an inner surface thereof to allow light emitted from the lamps 1 to be redirected toward a display part of the liquid crystal panel. Each of the lamps 1 is a cold cathode fluorescent lamp (CCFL), which includes electrodes (not shown) at both inner ends of the lamp tube to emit light when power is applied to the electrodes. The ends of each of the lamps 1 are fitted into grooves at both sides of the case frame 3. Both electrodes of each lamp 1 are connected to power input wires 9a and 9b, which are connected with a driving circuit via a separate connector (not shown), to transmit power for driving the lamp. Thus, each of the lamps 1 requires a separate connector. The power input wires 9a and 9b for a lamp are connected with a single connector in such a way that one of the power input wires 9a and 9b runs below the case frame 3 for connection with the connector.
FIGS. 2A and 2B are a plan view and a rear view of another related art direct light type backlight unit, respectively. As shown in FIGS. 2A and 2B, another related art backlight unit includes a plurality of external electrode fluorescent lamps (EEFLs) 11 arranged in constant intervals on a bottom cover 10. Each of the plurality of EEFLs 11 has first and second electrodes 12a and 12b at both ends on an outer surface of the lamp tube. First and second common electrodes 13a and 13b are respectively arranged at both ends of the plurality of EEFLs 11 to apply driving current to the first and second electrodes 12a and 12b of each EEFLs 11. First and second inverter PCBs 14a and 14b are disposed on a rear side of the bottom cover 10 to apply the driving current to the first and second common electrodes 13a and 13b. First and second transformers 15a and 15b respectively provide driving voltages to the first and second inverter PCBs 14a and 14b. First and second wires 17a and 17b are respectively connected between the first and second common electrodes 13a and 13b and the first and second transformers 15a and 15b. The first and second wires 17a and 17b are connected with the first and second transformers 17a and 17b via first and second connectors 16a and 16b, respectively.
FIG. 3 is a view illustrating a construction problem of the related art direct light type backlight unit shown in FIGS. 2A and 2B, and FIG. 4 is a view illustrating a waveform of voltage applied to common electrodes of the backlight unit shown in FIG. 3. In the related art backlight unit including the EEFLs as described above, the first and second electrodes 12a and 12b of each lamp 11 are connected in parallel with the first and second common electrodes 13a and 13b, and drive the first and second common electrodes 13a and 13b. Alternating current of the same waveform, as shown in FIG. 4, is applied to the first and second common electrodes 13a and 13b. 
When the current of the same waveform is applied to the first and second common electrodes 13a and 13b, and drives the first and second common electrodes 13a and 13b in parallel, a parasitic capacitance occurs between the bottom cover 10 and the lamps 11 at a lower end of the bottom cover 10 near the lower end of a liquid crystal display screen causing a current leakage. The parasitic capacitance unbalances the impedance of the lamps arranged at the lower end of the liquid crystal display screen, and thus deteriorates the brightness of the lamps at the lower end of the liquid crystal display device. Further, the backlight unit including CCFL or EEFLs as described above has a problem in that, when the backlight unit remains in darkness or at low temperatures for a long period of time, such as when the backlight unit remains in a non-driven state for a long period of time, initial start-up or re-driving of the lamps is difficult.